This invention relates generally to circuit protection apparatus and more particularly to an improved solid state trip circuit for controlling the operation of a circuit interrupter.
It has been known for many years that molded case circuit breakers may utilize a trip mechanism including both electromagnetic and electrothermal devices for initiating the mechanical separation of its current-carrying contacts in response to an excessive flow of current in the circuit protected by the circuit breaker. Although such circuit breakers have operated accurately and economically up to the present date, they have suffered from the shortcomings that they are not easily adjustable to change tripping levels or periods of time for response and that, therefore, they have not been easily coordinated with other circuit breakers in a multi-circuit breaker system to assure proper interplay between the various circuit breakers.
Within the last twenty years, various static overcurrent electronic relays such as those shown in U.S. Pat. Nos. 3,331,927 and 3,766,436 to Zocholl et al.; U.S. Pat. Nos. 3,573,555 and 3,327,171 to Lipnitz et al.; and U.S. Pat. No. 3,590,326 to Watson have been developed for use with large circuit breakers to electronically control the tripping of the circuit breaker. Such static overcurrent relays usually involved the use of high precision components and costly circuit design which rendered such circuits uneconomical for use in smaller circuit breakers.
However, in recent years, solid state trip circuits such as those shown in U.S. Pat. No. 3,818,275 to Shimp and U.S. Pat. No. 4,060,844 to Davis et al. have been developed specifically for use with molded case circuit breakers. These circuits have overcome the adjustability and coordination shortcomings of the aforementioned thermal-magnetic trip units, but have suffered from the shortcoming that lengthy calibration testing has been required during production to assure accurate tripping. Such calibration requirements have been both costly and time consuming.